MediaWiki currently still supports browsers with ES5, but it seems the ES6 keyword const has slipped into the code of page previews.
"./src/constants.js": function(e, t, n) {
"use strict";
var i = jQuery,
r = i.bracketedDevicePixelRatio && i.bracketedDevicePixelRatio() || 1;
const o = {
off: "off",
on: "on",
control: "control"
};
t.a = o, t.b = {
THUMBNAIL_SIZE: 300 * r,
EXTRACT_LENGTH: 525
}
},This causes fatal errors on older versions of Safari and IE.